Re: The speed of gravity revisited

On Jul 30, 2:44 am, luke.s...@xxxxxxxxxxxxxx wrote:
On Jul 29, 9:10 pm, "Tom Van Flandern" <to...@xxxxxxxxxxxxxxxx> wrote:

This replies to Steve Carlip, Hans de Vries, Ken S. Tucker, Tom Roberts,
and Martin Hogbin. REFERENCES appended at the end.

Steve Carlip writes:
[TomVF]: The direction of the source mass as sensed by an orbiting target
body is toward its true instantaneous position when the target body or
field point is at rest. And it is toward the source mass's retarded
position (retarded by the speed of gravitational force) when the target
body is orbiting. That's elementary physics. The exact same statement is
equally true if the source mass is moving, then stops (your example)..
That "move, then stop" distraction just makes a simple problem more
complicated.
[Carlip]: Tom, this is simply wrong. According to general relativity, when
the source mass stops, the acceleration of the test body will continue to
track its "extrapolated" motion, until a time equal to the light-travel
time from the source to the test body, at which point the acceleration
will rapidly swing back to the actual direction of the source mass. This
isn't a guess or an opinion. It's a calculation. Stop talking through your
hat, and do the math!

Steve, once again I insist, we have no issues with the math. None. This
discussion and the issues of importance are all about the interpretation of
the math, the physics behind the math, what the math means for reality. It
would be nice if we could begin getting our heads together about this. Can
you see your way to acknowledging that general relativity has two different
physical interpretations, the geometric and the field? That is not just my
opinion, but one well known through the early and middle 20th century,
suppressed only following the MTW bid to make the geometric interpretation
dominant. But major physicists as recent as Feynman reaffirmed this duality
of interpretation, reminding us that the geometric interpretation is not
really needed for anything in particular. [Ref. 1] All of relativity can be
described with classical forces in a Euclidean 3-space context, which is the
field interpretation of general relativity.

Or do you wish to redefine "general relativity" to exclude the field
interpretation? :-(

Once we agree on this indisputable fact that the geometric
interpretation is not the only way to understand the math of GR, then you
will readily see that my statement was true for the field interpretation,
and yours for the geometric interpretation. The field equations of GR
describe only the gravitational potential field, so the claims you make are
true for the potential field. Whether or not they apply also to force or
acceleration requires an additional assumption about how force relates to
potential. In the geometric interpretation, whatever is true for the field
is also true for gravitational force/acceleration (your view). But in the
field interpretation, force/acceleration drives the field, not the other way
around (my view).

The next step is to notice that what you claim (that gravitational
acceleration of a target body will suddenly stop tracking the instantaneous
position of the source mass if the source experiences a sudden acceleration)
violates the clear evidence from binary pulsars, which show that the
acceleration continues to track the instantaneous position of the source
mass even when the source mass accelerates by a substantial amount. You
apparently wish to claim that there will be a major difference in behavior
of the target body if the source mass acceleration experiences a "sudden"
(i.e., unanticipated) acceleration instead of just a "substantial"
acceleration. But you have no leg to stand on in making such a claim. Not a
shred of evidence supports it [Ref. 2]; the lifting of the universal speed
limit removes any logical reason to expect it [Ref. 3]; and it is also
untrue in electrodynamics, where sudden accelerations of charges are still
tracked essentially instantaneously [Ref. 4].

You learned only one way to interpret the math of GR. I learned two
ways. If you became more familiar with the way relativistic celestial
mechanics operates, I think you will be (pleasantly?) surprised. It opens up
a lot of doors to a deeper understanding of gravity, and settles a lot of
paradoxes, especially in QM.

and Hans de Vries writes:

[TomVF]: On the contrary, binary pulsars prove that when the source mass
accelerates (as in Steve's example), the target body responds almost
instantly.
[de Vries]: The EM case is much simpler with the essentially same result.
Did you ever try to derive the direction of the E-field from a moving
charge?

Of course. That was featured in my joint paper with J.P. Vigier [Ref.
3]. And it is indeed closely parallel with the gravitational case, where
even accelerations of charges are tracked instantaneously [Ref. 4], even
though many textbooks assert the opposite without any experimental support
for that assumption at all. Experimental evidence trumps textbook claims.

[de Vries]: I wrote a derivation of the Lienard-Wiechert potentials in
chapter 2 of my book which is step-by-step with comments ...

And I published a detailed explanation of why such retarded potentials
describe only the behavior of the potential field, but say nothing about the
forces (gravitational or electrodynamic) that shape potential fields. [Ref.
5]

As I said to Steve: Don't befuddle yourself by making this complicated.
Consider a static potential field around an unmoving source mass. If a
photon (speed c) and a classical graviton (speed undetermined) leave a
source mass at the same instant and travel the same linear path to the same
orbiting target body, the photon arrives from the retarded source direction
(naturally), whereas the classical graviton arrives from the instantaneous
source direction (unexpectedly). The only physical difference available to
account for this change in the action direction is that they travel the same
path with different speeds.

If you substitute "virtual photon" for "classical graviton" and "charge"
for "mass", the same reasoning applies to electrodynamics. Virtual photons,
as you might have heard, are alleged to have infinite speed to "explain"
this unexpected behavior.

and Ken S. Tucker writes:

[Tucker]: suppose the Sun was to instantly convert to light energy. Since
Earth is in free-fall, that event (aside from the flash) would be
undetectable by gravitation except by a tidal variation. ... Either Steve
or Tom may predict when that tidal variation could be detected.

My prediction is already on record. Force shuts off instantly (assuming
your "light energy" has no mass), whereas the field change takes 8.3 minutes
to arrive at Earth. See [Ref. 6].

[Tucker]: Suppose we examine the geodesy of a satellite in a normally
circular orbit, orbiting Earth. The gravitational effect of the Sun will
cause a tidal effect on the orbit of that satellite, such that the
circular orbit will be pulled to an ellipse. The semi-major axis of that
elliptical orbit will point to either the "absolute" position of the Sun
as Flandern predicts or it will point to the "apparent" position of the
Sun as Tucker and Carlip predict. The diff between those two predicted
axes is ~ 20" arc.

The solar eclipse experiment already showed precisely what your example
suggests as a test, where the Moon is the satellite in question. And the
answer is unambiguous and undisputed: the bulge in the Moon's orbit is
toward the instantaneous Sun, not the retarded Sun. [Ref. 2]

[Tucker]: The data base exists, 1st guy to tease out the data accurately
gets my nomination for a Nobel.

Thank you. ? But I'll be even happier if you do not change your test in
some way, now that it has gone in an unexpected direction for you. That is
what "controls against bias" is all about in scientific method - not
discounting test results when they go against expectations.

[Tucker]: What I'm pointing out to Steve and Tom is the means to
experimentally resolve this ongoing argument, prior to them sitting in an
old folks home throwing false teeth at each other.

It was a nice thought. But we still have to find some way to entertain
ourselves in the old folks home. And I'll throw my false teeth at you if you
don't get my last name right: "Van Flandern" (note capital "V"). ?

and Tom Roberts writes:
[Roberts]: The actual observations for EVERY ONE of the experiments TVF
cites are in accord with the predictions of GR.

Of course. No one has suggested otherwise. [shrug] But you still don't
get what the discussion is about.

[Roberts]: So when he says "binary pulsars prove that when the source mass
accelerates (as in Steve's example), the target body responds almost

instantly", you should recognize: ... the source NEVER accelerates "as in
Steve's example", the two objects merely orbit each other.

Now you are talking nonsense. Look up the definition of "accelerate" and
remember we are having a 3-space discussion. Anything in orbit is
accelerating by definition of the word: the time rate of change of (3-space)
velocity (a vector).

[Roberts]: In GR, nothing that carries energy, momentum, or information
can travel with local speed >c relative to any locally-inertial frame..
TVF's "speed of gravity" would violate this, IF IT WERE GR. He is wrong,
because he is not actually applying GR. That is Steve's point, and mine.

And you will remain of that unsupportable opinion all your life unless
you start to appreciate that there is a difference between math and the
physical interpretation of that math. Learn the field interpretation of GR.
Study some celestial mechanics. THEN let's talk.

As to the substance of your point, for the benefit of others uninhibited
by being...

read more >>

Thanks for your posts Tom et al.
I have been following this discussion for years.

Sorry to not address your points specifically, but my question is
perhaps simpler.
At first it appears that stable elliptic orbits are consistent with
superluminal speed of gravity field, as Van Flandern has presented in
his papers.

Stable elliptic orbits are also consistent with:

<< inductive coupling [which] takes place in the
near field ν± << c/r12 between coherently
coupled individual dipoles, through their red-shifted
local antipodal image. This allows the exchanged photons
to be virtual and the coherent modes to genuinely
belong to the coupled oscillators while ensuring that
the range of gravity spans the Universe. In this sense,
the Zitterbewegung of all matter near and far can be felt
here, in the far infrared (λred shifted > 1010 LYrs),
by the Zitterbewegung of a test charged particle or dipole,
in direct proportion to the rate of both, that is, to their
energy, owing to their common coherent modes with the
universe at large, through red-shifted tunneling photons.
This is in agreement with the equivalence principle. >>
http://arxiv.org/abs/physics/0107015v6
http://en.wikipedia.org/wiki/Induced_gravity

Hydrogen and helium is far more common than examples
of superluminal propagation.


Carlip's paper showed that orbits are also consistent
with light speed propagation, due to a subtlety of retarded potentials
also present in the Lienard-Wiechert potentials of electromagnetism.

Is there any experimenatal evidence, past or future, that can decide
between these two approaches? The speed of gravity being equal to
that of light fits in better with current theory and so unless there
is some real test between the interpretations that will remain the
more accepted version. However it would sure be nice to rule out
superluminal propagation.. would a change in the propagation speed
change the observable behavior of tight binary systems? It seems a
laboratory test is well beyond our current technology..

<< Atoms are massive particles. They are therefore
accelerated by gravity. A matter wave beam will fall
like a beam of ordinary atoms. >>
http://cua.mit.edu/ketterle_group/Projects_1997/atomlaser_97/atomlaser_comm..html

Sue...


Thanks -

.

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